162 Illinois State Laboratory of Natural Histoinf. 



When gills are present they are usually filamentous or 

 thread-like and borne on the sides or back. The spiracles 

 or stigmata are the external openings of the internal air- 

 tubes or tracheae. 



The separation of the pupae of this group has proven 

 difficult. The dipterous pupa when it develops within 

 the old larval skin usually gets air from spiracles at the 

 posterior end, but the free pupa, with legs and wing- 

 pads visible, receives its principal air supply from the 

 spiracles of the first thoracic segment (prothorax). 

 These are usually well up on the dorsal surface, lai-ge or 

 prominent, and very frequentl^'^ borne upon long antenna, 

 like prolongations. On the other hand, the prothoracic 

 spiracles in the remaining orders are on the sides at the 

 hinder edge of the segment and never very conspicuous. 

 The antennae of the pupa rest against the surface of 

 the body, but may be known by their origin on the 

 upper surface of the head. 



KEY TO THE ORDERS OF IMMATURE AQUATIC INSECTS. 



In this key extensive use has been made of the chapter 

 on Insects by I)r. E. Schmidt-Schwedt in "Die Tier- und 

 Pflanzenwelt des Stisswassers." The eggs can best be 

 determined at present b}'' an examination of the various 

 waj^s in which they are deposited, as illustrated by the 

 figures which will be published in the course of this 

 work. 



Nymphs (larvae and pupae) similar at all ages; wing- 

 pads present except on very young nymphs, 

 wanting in Thysanura; thoracic legs always 

 present and functional; no abdominal legs. ...A. 

 Larvae without wing-pads; pupae with wing-pads and 

 with thoracic legs visible but not in use;* pupa 

 sometimes concealed in a hardened footless 

 larval skin B. 



♦ The pupse of PhryKaneida; use their thoracic legs on leaving their cases for emer- 

 pence. Other exceptions to the characters given in these Iceys will probably appear as 

 our knowledge increases. They can only approximate the truth at present, but mav 

 serve as a stepping-stone to something better in the future. Although they are drawn 

 up for aquatic forms exclusively, it seems to me that the natural relationships of the- 

 different orders are here very unmistakably exhibited. 



